This invention relates to brushless D.C. motors used in a variety of applications. More particularly, this invention relates to a rotor magnet position sensor technique for such motors using RFID tags mounted on the magnets and associated tag reading elements.
Brushless D.C. motors are known and are finding increasing use in a wide variety of applications. Such motors rely on switching circuits in the electrical power distribution system to provide the necessary electrical power commutation required to operate the motor. The switching circuits rely on position feedback signals which indicate the rotational position of magnets mounted on the motor rotor to properly time the application of electrical power to the stator coils. The most widely implemented current type of rotor position feedback signal generator uses a plurality of rotor magnet position sensors, usually Hall effect sensors, to provide the necessary rotor magnet position feedback signals. FIG. 1 illustrates an example of a known brushless D.C. motor using Hall effect rotor magnet position sensors. As seen in this Fig., which is a schematic sectional view taken normal to the rotational axis of the motor, a rotor 11 is mounted for rotation in a direction suggested by arrow 12. A plurality of permanent magnets 13-1 . . . 13-4 is secured to the outer surface of rotor 11. The magnets 13-1 . . . 13-4 are arranged in alternating magnetic polarities, such that the north pole of magnet 13-1 is flanked by the south poles of magnets 13-2 and 13-4, the south pole of magnet 13-2 is flanked by the north poles of magnets 13-1 and 13-3, etc. Rotor 11 is concentrically mounted within a stator 15 fabricated from a magnetizable material, such as a laminated stack of steel plates, and having a plurality of pole teeth 16-1 . . . 16-3 and a corresponding plurality of axially extending slots 17-1 . . . 17-3. Power distribution coils 18-1 . . . 18-3 are individually wound about pole teeth 16-1 . . . 16-3, and these coils are connected in a star configuration to the electrical power source shown in FIG. 2. Rotor 11 is caused to rotate by the proper sequential application of electrical power to coils 18-1 . . . 18-3, which generate magnetic fields capable of interacting with the magnetic fields permanently generated by rotor magnets 13-1 . . . 13-4 to provide rotational force to rotor 11.
In the FIG. 1 embodiment, rotor magnet position feedback signals are provided by Hall effect sensors 19-1 . . . 19-3, which use the well-known Hall effect to generate electrical rotor magnet position feedback signals in response to the passage thereby of the magnetic fields generated by rotor magnets 13-1 . . . 13-4. As seen in FIG. 2, which is a schematic partial block diagram of the power switching and distribution circuitry for the brushless D.C. motor illustrated in FIG. 1, the rotor magnet position feedback signals generated by sensors 19-1 . . . 19-3 are coupled to a motor controller and driver unit 21. Unit 21 incorporates a microcontroller which processes these feedback signals and uses the positioning information contained therein to control the operation of a bank of power switching transistors 22-1 . . . 22-6 connected as shown, which are used to apply electrical power from a D.C. source (illustrated as a battery 24) to the stator coils 18-1 . . . 18-3. By properly sequencing the application of electrical power to coils 18-1 . . . 18-3, the rotor 11 is caused to rotate at the desired speed, thereby operating the motor. Further information regarding the structure, function and operating characteristics of brushless D.C. motors using Hall effect rotor magnet positioning sensors can be found in the following U.S. Patents, the disclosures of which are hereby incorporated by reference: U.S. Pat. No. 6,819,068 issued Nov. 16, 2004; U.S. Pat. No. 6,934,468 issued Aug. 23, 2005; U.S. Pat. No. 6,941,822 issued Sep. 13, 2005; and U.S. Pat. No. 6,954,042 issued Oct. 11, 2005.
Although brushless D.C. motors equipped with Hall effect rotor magnet position feedback sensors have been widely implemented in the past, certain disadvantages exist with this design configuration. Firstly, the operating characteristics of Hall effect sensors are temperature dependent. Consequently, in applications requiring more than a minimum of precision some means of compensating for the operating characteristics temperature dependency must be incorporated into the motor controller and driver unit 21. At the very least, this requires the addition of a temperature sensing element adjacent the Hall effect sensors, and special temperature compensation routines incorporated into the motor controller and driver unit 21. Further, Hall effect sensors do not operate reliably at elevated temperatures in excess of about 120 degrees Centigrade. While this temperature sensitivity does not adversely affect the operation and reliability of such sensors at relatively low temperatures within the reliable operating range (when combined with the temperature compensation routines), in many applications the environmental temperature to which the motor is subjected frequently exceeds 120 degrees Centigrade. In such a temperature environment, temperature compensation does not guarantee reliable operation of the Hall effect sensors. Consequently, either special cooling techniques must be incorporated into the brushless D.C. motor, or the Hall effect sensor design must be replaced by some other position signal feedback technique. Still further, Hall effect sensors do not function well in dirty environments regardless of temperature considerations, such as applications in which dusty or oily conditions are encountered. In such contaminated environments the only solution has been to periodically clean the interior of the motor.
Efforts to design a brushless D.C. motor having rotor magnet position feedback sensors devoid of the above-noted disadvantages have not been successful to date.